The present invention relates generally to the transmission of radio frequency signals through a dielectric wall (e.g. a vehicle window) and is illustrated in the context of a dual-band, glass mount mobile antenna system.
Window mounted antennas have been welcome for many years in mobile radio links, especially in 800 MHz cellular telephone service (sometimes known by the acronym "AMPS") due to their obvious advantages to the consumer. These advantages include the ease of installation and the fact that it is not necessary to drill a hole in the vehicle, which would detract from its value. Others include enhancing the signal strength for better communication quality, and moving radiation outside the vehicle. Much effort has been devoted to designing effective window mounted antenna systems for mobile radio links.
A new type of cellular service, known in the United States as PCS, is growing in popularity. This service occupies frequencies between 1500 and 2000 MHz. (In the United States, the PCS band is at 1900 MHz. In Europe this service (termed PCN) is at 1800 MHz. In Japan this service (termed PHS) is at 1500 MHz.). This alternate cellular service creates a potential compatibility problem with the existing, well-established 800 MHz cellular infrastructure. Many effort have been made to address these comparability issues. The most effective solution seems to be the emergence of multi-mode, multi-band handsets that automatically adapt to the service available in a given area. For example, Qualcomm offers a dual-band, dual mode phone known as the QCP-2700, which provides service over both the 800 MHz AMPS band and the 1900 MHz CDMA PCS band. Ericsson has similar offerings, such as its models PD 328 and PD 398, which each provides both AMPS and PCS service.
These dual-band handsets pose a significant engineering challenge, namely the design of a single antenna that provides good performance at both the AMPS and PCS bands. This challenge is compounded when the antenna is vehicle-mounted and fed through-glass. The through-glass coupling system must provide high efficiency coupling (and in some instances antenna matching) at both AMPS and PCS frequencies. Moreover, the bandwidth required at each band is large (e.g. up to 11% in the PCS bands), posing a further engineering obstacle.
A variety of through-glass feed techniques are known, as illustrated by the cited patents. Many are capacitively-coupled systems. Examples include U.S. Pat. No. 4,089,817 (Kirkendall, 1978), U.S. Pat. No. 4,839,660 (Hadzoglou, 1989), U.S. Pat. No. 4,992,800 (Parfitt), U.S. Pat. No. 4,857,939 (Shimazaki) and U.S. Pat. No. 4,785,305 (Shyu). In addition to capacitive coupling, these systems also generally employ LC impedance matching networks.
There are several problems with the foregoing designs. First the capacitive coupling patches cannot be large in comparison with the operating wavelength. Therefore; high impedance coupling (several hundred ohms) cannot be avoided. This leads to high loss due to the leakage of electrical field at high frequencies. Also, at high frequency bands like PCN/PCS, even a small patch no longer behaves as a lumped capacitor element. Due to the thickness of vehicle glass and various stray capacitances, such capacitive coupling circuits can bypass the signal and make it more difficult to match the (typically) high impedance of the antenna to a 50 ohm system. Additionally, the high impedance coupling creates a moisture sensitive structure. U.S. Pat. No. 4,764,773 (Larsen, 1988) describes a better coupling structure to improve performance in the presence of moisture, but it is still subject to the patch size limitation.
Design of a vehicle-mounted radiator also poses difficulties at PCS frequencies. Collinear array whips are desirable for mobile service due to their gain in the vertical plane. However, such whips do not have uniform current distribution. The lower section of the array has the highest current and produces the strongest radiation. But in most vehicle mounting arrangements the lower section of the whip is blocked by the vehicle roof, causing severe pattern distortion and deep nulls. This situation becomes worse at the 1.5-2 GHz PCN/PCS bands simply because the length of radiator is only half that at the 800 Mhz hand due to the doubling of the frequency.
Elevated-feed whips are sometimes employed to avoid the pattern distortion caused by vehicle roof blockage of radiation. But elevated-feed antennas are not readily matched for broadband operation (i.e. 11% for DCS-1800). Moreover, many such antennas, employing decoupling sleeve or slots, have low impedance feeds (e.g. 50 ohms). High impedance capacitive-feed systems thus pose large impedance transitions. Impedance transformation at PCS frequencies by use of conventional LC circuits is very inefficient due to the high loss of such circuits at these high frequencies.
U.S. Pat. No. Re.33,743 (Blaese) proposes a capacitively coupled antenna system for coupling a coaxial cable through glass to a low impedance quarter-wave whip. But in the PCS bands, the suggested antenna is only 1.7" long. Again, this is completely below the roof line of vehicle, causing severe pattern distortion and deep nulls.
To avoid some of the problems associated with capacitive coupling, a coupling arrangement employing resonant cavities has been proposed. U.S. Pat. No. 4,939,484 (Harada), for example, discloses a through-glass coupler employing a pair of tuned helix cavities. Unfortunately, the liarada cavity aperture must be sized to satisfy a 1/3 object frequency criterion, as described in the patent. That is, for 800 MHz, the helix should be designed for 266 MHz. The resulting cavity has a Q of over 1000 and sufficient coupling aperture. But at the 1.8 GHz band, the helix must be designed for 600 MHz. A 600 MHz helix cavity has a small aperture which is nearly half of the cellular band. A significant drop of unloaded Q is unavoidable due to the thin helix, and the coupling coefficient is not sufficient to provide an 11% bandwidth. Other drawbacks of such helix cavity couplers including highly critical tuning characteristics, and difficulties in mass production due to their complex 3D structure. Impedance matching is also difficult to implement in the cavity context.
In my U.S. Pat. No. 5,471,222, a pair of TE.sub.01.delta. high dielectric, constant-Q Ba-Bd-Ti oxide (ceramic) resonators were employed to overcome various problems of prior art PCS band through-glass couplers. This approach proved to be highly efficient, with insertion losses of only 0.5 dB through 5 mm automobile glass at 1.8 GHz. However, this arrangement proved sensitive to de-tuning in the field. Additionally, it suffered from a high manufacturing cost.
In my U.S. Pat. No. 5,451,966, a rectangular slot coupling scheme was employed to replace the expensive Ba-Nd-Ti Oxide ceramic. This arrangement built on the concept of dual-cavity coupling, where coupling is through an aperture.
The idea of slot coupling on an MSA (microstrip antenna) originated by Pozar. It provides a means to overcome the narrow band nature generally associated with MSAs. A "doggie bone"-shaped slot suggested by Pozar significantly increases the magnetic polarisability on the slot. This allows a short slot to achieve the necessary coupling while at the same time keeping backward emissions low.
Pozar and other researchers' work was basically limited to numerical solutions of the slot-fed microstrip antenna and multilayer arrays on a ground plane. But the bandwidth advantages of this type of MSA can be used to enhance the performance of the planar slot-cavity coupler.
In my above-referenced pending application, an annular ring aperture is employed for through-glass coupling. It is understood that in the rectangular slot design, the requirement for a tight coupling coefficient leads to an increase in slot length, which increases the level of backwards radiation. A major advantage of the annular ring aperture coupler over rectangular slot coupling is that it provides an increased coupling coefficient, which is extremely valuable for coupling through a thick dielectric wall. Another advantage is that the relatively radial distribution of E field on an annular ring aperture coupler successfully reduces the so-called "Microstrip Antenna Effect" in the rectangular slot approach. The annular ring is the complementary element to a small loop antenna. It is well known that a small loop antenna has a very low radiation resistance, and thus has a very low radiation efficiency. But the reduction of backwards radiation merited the tradeoff. Feeding was accomplished without any transition by connecting a coplanar waveguide line directly to the center resonant element.
More recently, I have improved the annular ring aperture coupler. That design, shown in the attached FIGS. 7A-7D, employs two small circuit boards 201, 202, one of which is single sided. Inside the vehicle, an annular ring 203 is still used, excited by a stub 204. The coaxial feedline (not shown) connects with its center connector soldered to end 205 of the stub, and its shield soldered to foil 206. Plated-through holes 210 connect foil 206 to the groundplane 207 on the opposite side of the inside board 201. On the outside of the vehicle glass, however, the circuit board defines a loaded microstrip 208, to which the whip antenna attaches at end 209. The periphery 211 around the microstrip 208 is foil. A matching function is provided by the microstrip; no additional circuitry is required. The outer surface of the outside circuit board has no foil; just a hole through which the whip antenna connects to end 209.
Some of the evolution in recent high-frequency couplers, and their attendant decrease in transmission loss, is shown by the following:
For Rectangular slot, the transmission loss are accumulated as:
cable---microstrip--- slot---glass---slot---microstrip---i.m.n.---antenna. PA0 cable---microstrip--- annular ring ---glass--- annular ring ---i.m.n.---antenna. PA0 cable---microstrip---annular ring---glass--- loaded microstrip--- antenna.(integrated i.m.n.)
For annular ring aperture coupler, the transmission loss are accumulated as:
For the most recent work on annular ring, the transmission loss are accumulated as:
Where i.m.n represents impedance matching network.
As evidenced by the foregoing, there are numerous approaches for through-glass coupling at high frequencies. However, such approaches uniformly operate over a single, limited frequency band. The aperture coupled designs probably has the widest bandwidth, but even this is much less than one octave. For AMPS/PCS dual-band operation in the United State, the lowest frequency is 824 MHz and the highest is 1990 MHz, yielding a ratio of 2.415. Even in Europe, the ratio is still 2.112.
In accordance with a preferred embodiment of the present invention, a nonhomogenous quasi-TEM mode transmission line directional coupler arrangement is adapted to serve as a dual-band through-glass coupler. Such a directional coupler has four ports, but two are left open-circuited. By this arrangement, the signal fed by coaxial cable to one port is re-directed across the coupler to the diagonal port, which connects to the external antenna. The even and odd mode impedances of the coupling device are selected so that an over-coupled 3 dB coupler is realized; the two crossover points are located at the centers of the two frequency bands of interest.
This arrangement features very high efficiency since it is a complete distributed design, with no LC circuit elements. Other advantages include its low impedance coupling, and broadband behavior. Moreover, backwards radiation is substantially avoided while maintaining a high coupling coefficient. The coupler is mechanically rugged, easy to manufacture and inexpensive to produce.
A dual-resonant whip antenna or coplanar waveguide dipole type antenna is desirably connected to the coupler, thereby achieving a dual-band glass-mounted antenna system.
The foregoing and other features and advantages will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.
These objectives are accomplished in the present invention by implementing the quasi-TEM mode transmission line coupler with proper termination, providing an antenna with collinear elements while preserving the performance of the previous arts at the same time.